Reduction of plasma triglycerides in apolipoprotein C-II transgenic mice overexpressing lipoprotein lipase in muscle

LPL and its specific physiological activator, apolipoprotein C-II (apoC-II), regulate the hydrolysis of triglycerides (TGs) from circulating TG-rich lipoproteins. Previously, we developed a skeletal muscle-specific LPL transgenic mouse that had lower plasma TG levels. ApoC-II transgenic mice develop hypertriglyceridemia attributed to delayed clearance. To investigate whether overexpression of LPL could correct this apoC-II-induced hypertriglyceridemia, mice with overexpression of human apoC-II (CII) were cross-bred with mice with two levels of muscle-specific human LPL overexpression (LPL-L or LPL-H). Plasma TG levels were 319 +/- 39 mg/dl in CII mice and 39 +/- 5 mg/dl in wild-type mice. Compared with CII mice, apoC-II transgenic mice with the higher level of LPL overexpression (CIILPL-H) had a 50% reduction in plasma TG levels (P = 0.013). Heart LPL activity was reduced by approximately 30% in mice with the human apoC-II transgene, which accompanied a more modest 10% decrease in total LPL protein. Overexpression of human LPL in skeletal muscle resulted in dose-dependent reduction of plasma TGs in apoC-II transgenic mice. Along with plasma apoC-II concentrations, heart and skeletal muscle LPL activities were predictors of plasma TGs. These data suggest that mice with the human apoC-II transgene may have alterations in the expression/activity of endogenous LPL in the heart. Furthermore, the decrease of LPL activity in the heart, along with the inhibitory effects of excess apoC-II, may contribute to the hypertriglyceridemia observed in apoC-II transgenic mice.     

ANGPTL3 blockade with a human monoclonal antibody reduces plasma lipids in dyslipidemic mice and monkeys

Angiopoietin-like protein 3 (ANGPTL3) is a circulating protein synthesized exclusively in the liver that inhibits LPL and endothelial lipase (EL), enzymes that hydrolyze TGs and phospholipids in plasma lipoproteins. Here we describe the development and testing of a fully human monoclonal antibody (REGN1500) that binds ANGPTL3 with high affinity. REGN1500 reversed ANGPTL3-induced inhibition of LPL activity in vitro. Intravenous administration of REGN1500 to normolipidemic C57Bl/6 mice increased LPL activity and decreased plasma TG levels by ≥50%. Chronic administration of REGN1500 to dyslipidemic C57Bl/6 mice for 8 weeks reduced circulating plasma levels of TG, LDL-cholesterol (LDL-C), and HDL-cholesterol (HDL-C) without any changes in liver, adipose, or heart TG contents. Studies in EL knockout mice revealed that REGN1500 reduced serum HDL-C through an EL-dependent mechanism. Finally, administration of a single dose of REGN1500 to dyslipidemic cynomolgus monkeys caused a rapid and pronounced decrease in plasma TG, nonHDL-C, and HDL-C. REGN1500 normalized plasma TG levels even in monkeys with a baseline plasma TG greater than 400 mg/dl. Collectively, these data demonstrate that neutralization of ANGPTL3 using REGN1500 reduces plasma lipids in dyslipidemic mice and monkeys, and thus provides a potential therapeutic agent for treatment of patients with hyperlipidemia.     

Accelerated Fatty Acid Oxidation in Muscle Averts Fasting-induced Hepatic Steatosis in SJL/J Mice

The accumulation of triglycerides (TG) in the liver, designated hepatic steatosis, is characteristically associated with obesity and insulin resistance, but it can also develop after fasting. Here, we show that fasting-induced hepatic steatosis is under genetic control in inbred mice. After a 24-h fast, C57BL/6J mice and SJL/J mice both lost more than 20% of body weight and ∼60% of total body TG. In C57BL/6J mice, TG accumulated in liver, producing frank steatosis. In striking contrast, SJL/J mice failed to accumulate any hepatic TG even though they lost nearly as much adipose tissue mass as the C57BL/6J mice. Mice from five other inbred strains developed fasting-induced steatosis like the C57BL/6J mice. Measurements of the uptake of free fatty acids (FA) in vivo and in vitro demonstrated that SJL/J mice were protected from steatosis because their heart and skeletal muscle took up and oxidized twice as much FA as compared with C57BL/6J mice. As a result of this muscle diversion, serum-free FA and ketone bodies rose much less after fasting in SJL/J mice as compared with C57BL/6J mice. When livers of SJL/J and C57BL/6J mice were perfused with similar concentrations of FA, the livers took up and esterified similar amounts. We conclude that SJL/J mice express one or more variant genes that lead to enhanced FA uptake and oxidation in muscle, thereby sparing the liver from FA overload in the fasting state.Liver and adipose tissue coordinate metabolic responses to oscillations in nutrient availability (1, 2). In the postprandial state, the liver secretes triglycerides (TG)⁴ into the blood in very low-density lipoproteins (VLDL). In adipose tissue, lipoprotein lipase hydrolyzes the TG, producing fatty acids (FA) and monoglycerides that enter fat cells for reesterification and storage as TG (1). The activity of adipose tissue lipoprotein lipase is enhanced by the postprandial rise in insulin. At the same time, insulin inhibits lipolysis of stored TG in fat cells, assuring that the TG will be retained in the cells (3).Under fasting conditions, insulin falls and the inhibitory effect of insulin on adipose tissue lipolysis is diminished. The released FA enters the blood and is used as an energy source in liver, heart, and skeletal muscle. In the liver, excess FA are either re-esterified into TG for intracellular storage or oxidized and secreted as ketone bodies, which become the main energy source for the brain. In skeletal muscle during fasting, FA are oxidized to CO₂ (1, 2).We (4–6) and others (7) previously reported that livers of mice accumulate large amounts of TG after fasting for 6–24 h. In the current study, we screened 7 strains of inbred mice to study the genetic control of fasting-induced hepatic TG accumulation. Mice from 6 of 7 strains exhibited fasting-induced fatty liver. In the unique mouse strain (SJL/J), hepatic TG failed to accumulate after a 24-h fast even though the SJL/J mice lost amounts of body weight and adipose tissue that were similar to those of the other 6 strains. To trace the mechanism for the difference in hepatic TG accumulation, we conducted extensive comparisons of SJL/J mice and C57BL/6J mice. We provide evidence that mice from both strains release comparable amounts of FA from adipose tissue into blood after fasting. In the SJL/J mice, the bulk of these FA are taken up by muscle and oxidized. In C57BL/6J mice, FA uptake in muscle is comparatively low, and the excess FA are taken up by the liver where they are converted to TG. Thus, genetic control of muscle FA uptake determines the level of hepatic TG accumulation in fasted mice.     

Effect of chronic intermittent hypoxia on triglyceride uptake in different tissues

Chronic intermittent hypoxia (CIH) inhibits plasma lipoprotein clearance and adipose lipoprotein lipase (LPL) activity in association with upregulation of an LPL inhibitor angiopoietin-like protein 4 (Angptl4). We hypothesize that CIH inhibits triglyceride (TG) uptake via Angptl4 and that an anti-Angptl4-neutralizing antibody would abolish the effects of CIH. Male C57BL/6J mice were exposed to four weeks of CIH or intermittent air (IA) while treated with Ab (30 mg/kg ip once a week). TG clearance was assessed by [H³]triolein administration retroorbitally. CIH delayed TG clearance and suppressed TG uptake and LPL activity in all white adipose tissue depots, brown adipose tissue, and lungs, whereas heart, liver, and spleen were not affected. CD146+ CD11b− pulmonary microvascular endothelial cells were responsible for TG uptake in the lungs and its inhibition by CIH. Antibody to Angptl4 decreased plasma TG levels and increased TG clearance and uptake into adipose tissue and lungs in both control and CIH mice to a similar extent, but did not reverse the effects of CIH. The antibody reversed the effects of CIH on LPL in adipose tissue and lungs. In conclusion, CIH inactivates LPL by upregulating Angptl4, but inhibition of TG uptake occurs predominantly via an Angptl4/LPL-independent mechanism.     

Chronic high-fat feeding impairs adaptive induction of mitochondrial fatty acid combustion-associated proteins in brown adipose tissue of mice

Since brown adipose tissue (BAT) is involved in thermogenesis using fatty acids as a fuel, BAT activation is a potential strategy for treating obesity and diabetes. However, whether BAT fatty acid combusting capacity is preserved in these conditions has remained unclear. We therefore evaluated expression levels of fatty acid oxidation-associated enzymes and uncoupling protein 1 (Ucp1) in BAT by western blot using a diet-induced obesity C57BL/6J mouse model. In C57BL/6J mice fed a high-fat diet (HFD) over 2–4 weeks, carnitine palmitoyltransferase 2 (Cpt2), acyl-CoA thioesterase (Acot) 2, Acot11 and Ucp1 levels were significantly increased compared with baseline and control low-fat diet (LFD)-fed mice. Similar results were obtained in other mouse strains, including ddY, ICR and KK-Ay, but the magnitudes of the increase in Ucp1 level were much smaller than in C57BL/6J mice, with decreased Acot11 levels after HFD-feeding. In C57BL/6J mice, increased levels of these mitochondrial proteins declined to near baseline levels after a longer-term HFD-feeding (20 weeks), concurrent with the accumulation of unilocular, large lipid droplets in brown adipocytes. Extramitochondrial Acot11 and acyl-CoA oxidase remained elevated. Treatment of mice with Wy-14,643 also increased these proteins, but was less effective than 4 week-HFD, suggesting that mechanisms other than peroxisome proliferator-activated receptor α were also involved in the upregulation. These results suggest that BAT enhances its fatty acid combusting capacity in response to fat overload, however profound obesity deprives BAT of the responsiveness to fat, possibly via mitochondrial alteration.     

ApoAV reduces plasma triglycerides by inhibiting very low density lipoprotein-triglyceride (VLDL-TG) production and stimulating lipoprotein lipase-mediated VLDL-TG hydrolysis

ApoAV has been discovered recently as a novel modifier of triglyceride (TG) metabolism, but the pathways involved are currently unknown. To gain insight into the function of apoAV, adenovirus-mediated gene transfer of murine apoa5 to C57Bl/6 mice was employed. The injection of low doses of Ad-apoa5 (1-5 x 10(8) plaqueforming units/mouse) dose-dependently reduced plasma very low density lipoprotein (VLDL)-TG levels. First, we evaluated whether a reduced hepatic VLDL production contributed to the TG-lowering effect. Ad-apoa5 treatment dose-dependently diminished (29-37%) the VLDL-TG production rate without affecting VLDL particle production, suggesting that apoAV impairs the lipidation of apoB. Second, Ad-apoa5 treatment dose-dependently reduced (68-88%) the postprandial hypertriglyceridemia following an intragastric fat load, suggesting that apoAV also stimulates the lipoprotein lipase (LPL)-dependent clearance of TG-rich lipoproteins. Indeed, recombinant apoAV was found to dose-dependently stimulate LPL activity up to 2.3-fold in vitro. Accordingly, intravenously injected VLDL-like TG-rich emulsions were cleared at an accelerated rate concomitant with the increased uptake of emulsion TG-derived fatty acids by skeletal muscle and white adipose tissue in Ad-apoa5-treated mice. From these data, we conclude that apoAV is a potent stimulator of LPL activity. Thus, apoAV lowers plasma TG by both reducing the hepatic VLDL-TG production rate and by enhancing the lipolytic conversion of TG-rich lipoproteins.     

The VLDL receptor plays a major role in chylomicron metabolism by enhancing LPL-mediated triglyceride hydrolysis

The VLDL receptor (VLDLr) is involved in tissue delivery of VLDL-triglyceride (TG)-derived FFA by facilitating the expression of lipoprotein lipase (LPL). However, vldlr-/- mice do not show altered plasma lipoprotein levels, despite reduced LPL expression. Because LPL activity is crucial in postprandial lipid metabolism, we investigated whether the VLDLr plays a role in chylomicron clearance. Fed plasma TG levels of vldlr-/- mice were 2.5-fold increased compared with those of vldlr+/+ littermates (1.20 +/- 0.37 mM vs. 0.47 +/- 0.18 mM; P < 0.001). Strikingly, an intragastric fat load led to a 9-fold increased postprandial TG response in vldlr-/- compared with vldlr+/+ mice (226 +/- 188 mM/h vs. 25 +/- 11 mM/h; P < 0.05). Accordingly, the plasma clearance of [3H]TG-labeled protein-free chylomicron-mimicking emulsion particles was delayed in vldlr-/- compared with vldlr+/+ mice (half-life of 12.0 +/- 2.6 min vs. 5.5 +/- 0.9 min; P < 0.05), with a 60% decreased uptake of label into adipose tissue (P < 0.05). VLDLr deficiency did not affect the plasma half-life and adipose tissue uptake of albumin-complexed [14C]FFA, indicating that the VLDLr facilitates postprandial LPL-mediated TG hydrolysis rather than mediating FFA uptake. We conclude that the VLDLr plays a major role in the metabolism of postprandial lipoproteins by enhancing LPL-mediated TG hydrolysis.     

Apolipoprotein CI causes hypertriglyceridemia independent of the very-low-density lipoprotein receptor and apolipoprotein CIII in mice

We have recently shown that the predominant hypertriglyceridemia in human apolipoprotein C1 (APOC1) transgenic mice is mainly explained by apoCI-mediated inhibition of the lipoprotein lipase (LPL)-dependent triglyceride (TG)-hydrolysis pathway. Since the very-low-density lipoprotein receptor (VLDLr) and apoCIII are potent modifiers of LPL activity, our current aim was to study whether the lipolysis-inhibiting action of apoCI would be dependent on the presence of the VLDLr and apoCIII in vivo. Hereto, we employed liver-specific expression of human apoCI by using a novel recombinant adenovirus (AdAPOC1). In wild-type mice, moderate apoCI expression leading to plasma human apoCI levels of 12-33 mg/dl dose-dependently and specifically increased plasma TG (up to 6.6-fold, P < 0.001), yielding the same hypertriglyceridemic phenotype as observed in human APOC1 transgenic mice. AdAPOC1 still increased plasma TG in vldlr(-/-) mice (4.1-fold, P < 0.001) and in apoc3(-/-) mice (6.8-fold, P < 0.001) that were also deficient for the low-density lipoprotein receptor (LDLr) and LDLr-related protein (LRP) or apoE, respectively. Thus, irrespective of receptor-mediated remnant clearance by the liver, liver-specific expression of human apoCI causes hypertriglyceridemia in the absence of the VLDLr and apoCIII. We conclude that apoCI is a powerful and direct inhibitor of LPL activity independent of the VLDLr and apoCIII.     

ApoE2-associated hypertriglyceridemia is ameliorated by increased levels of apoA-V but unaffected by apoC-III deficiency

Apolipoprotein E2 (apoE2)-associated hyperlipidemia is characterized by a disturbed clearance of apoE2-enriched VLDL remnants. Because excess apoE2 inhibits LPL-mediated triglyceride (TG) hydrolysis in vitro, we investigated whether direct or indirect stimulation of LPL activity in vivo reduces the apoE2-associated hypertriglyceridemia. Here, we studied the role of LPL and two potent modifiers, the LPL inhibitor apoC-III and the LPL activator apoA-V, in APOE2-knockin (APOE2) mice. Injection of heparin in APOE2 mice reduced plasma TG by 53% and plasma total cholesterol (TC) by 18%. Adenovirus-mediated overexpression of LPL reduced plasma TG by 85% and TC by 40%. Both experiments indicate that the TG in apoE2-enriched particles is a suitable substrate for LPL. Indirect activation of LPL activity via deletion of Apoc3 in APOE2 mice did not affect plasma TG levels, whereas overexpression of Apoa5 in APOE2 mice did reduce plasma TG by 81% and plasma TC by 41%. In conclusion, the hypertriglyceridemia in APOE2 mice can be ameliorated by the direct activation of LPL activity. Indirect activation of LPL via overexpression of apoA-V does, whereas deletion of apoC-III does not, affect the plasma TGs in APOE2 mice. These data indicate that changes in apoA-V levels have a dominant effect over changes in apoC-III levels in the improvement of APOE2-associated hypertriglyceridemia.     

Glycosylphosphatidylinositol-specific phospholipase D influences triglyceride-rich lipoprotein metabolism

Glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) is a minor HDL-associated protein. Because many minor HDL-associated proteins exchange between different lipoprotein classes during the postprandial state and are also involved in triglyceride (TG) metabolism, we hypothesized that GPI-PLD may play a role in the metabolism of TG-rich lipoproteins. To test this hypothesis, we examined the distribution of GPI-PLD among lipoprotein classes during a fat tolerance test in C57BL/6 and LDL receptor-deficient (LDLR(-/-)) mice fed either a chow or high-fructose diet. In the fasting state in wild-type mice fed a chow diet, GPI-PLD was only present in HDL, whereas in LDLR(-/-) mice GPI-PLD was present in HDL and intermediate-density lipoproteins (IDL)/LDL. During the fat tolerance test, there was no change in total serum GPI-PLD levels in either model; however, a significant amount of GPI-PLD appeared in both VLDL (0.5-1% of total GPI-PLD) and IDL/LDL (5-10% of total GPI-PLD) in both models. The high-fructose diet increased both fasting and postprandial TG and serum GPI-PLD levels in both strains as well as the amount of GPI-PLD in VLDL. To determine whether GPI-PLD plays a direct role in TG metabolism, we increased liver GPI-PLD expression in C57BL/6 mice by adenovirus-mediated gene transfer, which resulted in a sevenfold increase in serum GPI-PLD levels. This change was associated with an increase in fasting (30%) and postprandial TG (50%) and a twofold reduction in TG-rich lipoprotein catabolism compared with saline or control adenovirus-treated mice. These studies demonstrate that GPI-PLD affects serum TG levels by altering catabolism of TG-rich lipoproteins.     

Adipose tissue glycerokinase activity in genetic and acquired obesity in rats and mice.

Glycerokinase activity in isolated fat cells was elevated in both Ob/Ob and Db/Db mice in comparison to their lean controls and this elevation was associated with obesity, hyperinsulinemia and hyperglycemia. In the other forms of acquired and genetic obesity in the rats and mice studied (also associated with hyperinsulinemia), adipose tissue glycerokinase activity was not elevated in comparison to lean control groups when expressed on a mg protein basis. It is concluded that the elevated glycerokinase activity is not due to the specific Db or Ob mutation, but is secondary to the obesity and hyperinsulinemia interacting with the similar genetic background in the C57BL/KsJ and the C57BL/6J mouse strains.     

ApoC-III deficiency prevents hyperlipidemia induced by apoE overexpression

Adenovirus-mediated overexpression of human apolipoprotein E (apoE) induces hyperlipidemia by stimulating the VLDL-triglyceride (TG) production rate and inhibiting the LPL-mediated VLDL-TG hydrolysis rate. Because apoC-III is a strong inhibitor of TG hydrolysis, we questioned whether Apoc3 deficiency might prevent the hyperlipidemia induced by apoE overexpression in vivo. Injection of 2 x 10(9) plaque-forming units of AdAPOE4 caused severe combined hyperlipidemia in Apoe-/- mice [TG from 0.7 +/- 0.2 to 57.2 +/- 6.7 mM; total cholesterol (TC) from 17.4 +/- 3.7 to 29.0 +/- 4.1 mM] that was confined to VLDL/intermediate density lipoprotein-sized lipoproteins. In contrast, Apoc3 deficiency resulted in a gene dose-dependent reduction of the apoE4-associated hyperlipidemia (TG from 57.2 +/- 6.7 mM to 21.2 +/- 18.5 and 1.5 +/- 1.4 mM; TC from 29.0 +/- 4.1 to 16.4 +/- 9.8 and 2.3 +/- 1.8 mM in Apoe-/-, Apoe-/-.Apoc3+/-, and Apoe-/-.Apoc3-/- mice, respectively). In both Apoe-/- mice and Apoe-/-.Apoc3-/- mice, injection of increasing doses of AdAPOE4 resulted in up to a 10-fold increased VLDL-TG production rate. However, Apoc3 deficiency resulted in a significant increase in the uptake of TG-derived fatty acids from VLDL-like emulsion particles by white adipose tissue, indicating enhanced LPL activity. In vitro experiments showed that apoC-III is a more specific inhibitor of LPL activity than is apoE. Thus, Apoc3 deficiency can prevent apoE-induced hyperlipidemia associated with a 10-fold increased hepatic VLDL-TG production rate, most likely by alleviating the apoE-induced inhibition of VLDL-TG hydrolysis.     

Endogenous apoC-I increases hyperlipidemia in apoE-knockout mice by stimulating VLDL production and inhibiting LPL

Previous studies have shown that overexpression of human apolipoprotein C-I (apoC-I) results in moderate hypercholesterolemia and severe hypertriglyceridemia in mice in the presence and absence of apoE. We assessed whether physiological endogenous apoC-I levels are sufficient to modulate plasma lipid levels independently of effects of apoE on lipid metabolism by comparing apolipoprotein E gene-deficient/apolipoprotein C-I gene-deficient (apoe-/-apoc1-/-), apoe-/-apoc1+/-, and apoe-/-apoc1+/+ mice. The presence of the apoC-I gene-dose-dependently increased plasma cholesterol (+45%; P < 0.001) and triglycerides (TGs) (+137%; P < 0.001), both specific for VLDL. Whereas apoC-I did not affect intestinal [3H]TG absorption, it increased the production rate of hepatic VLDL-TG (+35%; P < 0.05) and VLDL-[35S]apoB (+39%; P < 0.01). In addition, apoC-I increased the postprandial TG response to an intragastric olive oil load (+120%; P < 0.05) and decreased the uptake of [3H]TG-derived FFAs from intravenously administered VLDL-like emulsion particles by gonadal and perirenal white adipose tissue (WAT) (-34% and -25%, respectively; P < 0.05). As LPL is the main enzyme involved in the clearance of TG-derived FFAs by WAT, and total postheparin plasma LPL levels were unaffected, these data demonstrate that endogenous apoC-I suffices to attenuate the lipolytic activity of LPL. Thus, we conclude that endogenous plasma apoC-I increases VLDL-total cholesterol and VLDL-TG dose-dependently in apoe-/- mice, resulting from increased VLDL particle production and LPL inhibition.     

Hyperlipidemia and lipid peroxidation are dependent on the severity of chronic intermittent hypoxia.

Obstructive sleep apnea (OSA) is characterized by chronic intermittent hypoxia (CIH) and associated with dysregulation of lipid metabolisms and atherosclerosis. Causal relationships between OSA and metabolic abnormalities have not been established because of confounding effects of underlying obesity. The goal of the study was to determine if CIH causes lipid peroxidation and dyslipidemia in the absence of obesity and whether the degrees of dyslipidemia and lipid peroxidation depend on the severity of hypoxia. Lean C57BL/6J mice were exposed to CIH for 4 wk with a fractional inspired O2 (FI(O2)) nadir of either 10% (moderate CIH) or 5% (severe CIH). Mice exposed to severe CIH exhibited significant increases in fasting serum levels of total cholesterol (129 +/- 2.9 vs. 113 +/- 2.8 mg/dl in control mice, P < 0.05) and low-density lipoprotein cholesterol (85.7 +/- 8.9 vs. 56.4 +/- 9.7 mg/dl, P < 0.05) in conjunction with a 1.5- to 2-fold increase in lipoprotein secretion, and upregulation of hepatic stearoyl coenzyme A desaturase 1 (SCD-1). Severe CIH also markedly increased lipid peroxidation in the liver (malondialdehyde levels of 94.4 +/- 5.4 vs. 57.4 +/- 5.2 nmol/mg in control mice, P < 0.001). In contrast, moderate CIH did not induce hyperlipidemia or change in hepatic SCD-1 levels but did cause lipid peroxidation in the liver at a reduced level relative to severe CIH. In conclusion, CIH leads to hypercholesterolemia and lipid peroxidation in the absence of obesity, and the degree of metabolic dysregulation is dependent on the severity of the hypoxic stimulus.     

Variation in Type 2 Diabetes-Related Phenotypes among Apolipoprotein E-Deficient Mouse Strains

We recently have found that apolipoprotein E-deficient (Apoe^-/-) mice with the C57BL/6 background develop type 2 diabetes when fed a Western diet for 12 weeks. In the present study we constructed multiple Apoe^-/- mouse strains to find diabetes-related phenotyptic variations that might be linked to atherosclerosis development. Evaluation of both early and advanced lesion formation in aortic root revealed that C57BL/6, SWR/J, and SM/J Apoe^-/- mice were susceptible to atherosclerosis and that C3H/HeJ and BALB/cJ Apoe^-/- mice were relatively resistant. On a chow diet, fasting plasma glucose varied among strains with C3H/HeJ having the highest (171.1 ± 9.7 mg/dl) and BALB/cJ the lowest level (104.0 ± 6.6 mg/dl). On a Western diet, fasting plasma glucose rose significantly in all strains, with C57BL/6, C3H/HeJ and SWR/J exceeding 250 mg/dl. BALB/cJ and C3H/HeJ were more tolerant to glucose loading than the other 3 strains. C57BL/6 was sensitive to insulin while other strains were not. Non-fasting blood glucose was significantly lower in C3H/HeJ and BALB/cJ than C57BL/6, SM/J, and SWR/J. Glucose loading induced the 1st and the 2nd phase of insulin secretion in BALB/cJ, but the 2nd phase was not observed in other strains. Morphological analysis showed that BALB/cJ had the largest islet area (1,421,493 ± 61,244 μm²) and C57BL/6 had the smallest one (747,635 ± 41,798 μm²). This study has demonstrated strain-specific variations in the metabolic and atherosclerotic phenotypes, thus laying the basis for future genetic characterization.     

BSB: A New Mouse Model of Multigenic Obesity

We report here a new mouse model of multigenic obesity. Backcross progeny ((C57BL/6J x Mus spretus)F1 x C57BL/6J), designated as BSB mice, range from 1% to 50% body fat. Since both parental strains are relatively lean, the wide range of the phenotype in the BSB mice indicates the involvement of multiple genes to produce obesity. Obesity in BSB mice results from increases in both intraabdominal and subcutaneous fat and is associated with hyperinsulinemia, hyperglycemia, and hyperlipidemia. Female and male BSB mice do not differ in the degree of obesity obtained. Stimulated plasma corticosterone levels are reduced in obese male and female mice. The development of appropriate genetic markers and statistical methods have made it feasible to analyze quantitative polygenic traits in animal models by employing F2 or backcross progeny. Thus, this BSB model is uniquely suited to the genetic analysis of the multifactorial quantitative trait of obesity and its associated phenotypes. (OBESITY RESEARCH 1993;1:271–280)     

DHA attenuates postprandial hyperlipidemia via activating PPARα in intestinal epithelial cells

It is known that peroxisome proliferator-activated receptor (PPAR)α, whose activation reduces hyperlipidemia, is highly expressed in intestinal epithelial cells. Docosahexaenoic acid (DHA) could improve postprandial hyperlipidemia, however, its relationship with intestinal PPARα activation is not revealed. In this study, we investigated whether DHA can affect postprandial hyperlipidemia by activating intestinal PPARα using Caco-2 cells and C57BL/6 mice. The genes involved in fatty acid (FA) oxidation and oxygen consumption rate were increased, and the secretion of triacylglyceride (TG) and apolipoprotein B (apoB) was decreased in DHA-treated Caco-2 cells. Additionally, intestinal FA oxidation was induced, and TG and apoB secretion from intestinal epithelial cells was reduced, resulting in the attenuation of plasma TG and apoB levels after oral administration of olive oil in DHA-rich oil-fed mice compared with controls. However, no increase in genes involved in FA oxidation was observed in the liver. Furthermore, the effects of DHA on intestinal lipid secretion and postprandial hyperlipidemia were abolished in PPARα knockout mice. In conclusion, the present work suggests that DHA can inhibit the secretion of TG from intestinal epithelial cells via PPARα activation, which attenuates postprandial hyperlipidemia.     

Differential expression of lipoprotein lipase gene in tissues of the rat model with visceral obesity and postprandial hyperlipidemia

Postprandial hyperlipidemia is frequently accompanied with intra-abdominal visceral accumulation in human subjects. We have found that the decreased lipoprotein lipase (LPL) mass and activity is negatively associated with the amount of visceral fat accumulation. Here, we studied the postprandial hyperlipidemia using the OLETF rat, a model with visceral obesity, in order to clarify the molecular mechanism causing postprandial hyperlipidemia accompanied with visceral obesity. At the same age of 32 weeks, the OLETF rats showed obviously higher plasma leptin, total cholesterol, triglyceride, and HDL-cholesterol levels than the control LETO rats, although the plasma glucose level was not significantly different. Fat-loading test revealed the delayed metabolism of exogenous fat in the OLETF rats compared to the LETO rats, similar to human subjects with visceral obesity. In the obese rats, plasma levels of LPL mass and activities were 60 and 49% of control rats. The expression of LPL gene was decreased in subcutaneous adipose tissues and skeletal muscle of OLETF rats to 40 and 52% compared to those of LETO rats. In OLETF rats, plasma tumor necrosis factor-alpha (TNF-alpha) and insulin levels were increased to 2.0- and 2.3-folds compared to those in control rats. Furthermore, plasma insulin and TNF-alpha levels in OLETF rats were negatively correlated with the expression levels of LPL gene in subcutaneous fat and muscle. These results indicate that decreased LPL mass and activity in the animal model with visceral obesity is possibly caused by decreased expression of LPL gene in tissues mediated by the increased levels of insulin and TNF-alpha. The different expression of LPL gene in tissues associated with the increased levels of insulin and TNF-alpha possibly elucidate the underlying mechanisms involving the postprandial hyperlipidemia observed in visceral obesity.     

Severe hypertriglyceridemia in human APOC1 transgenic mice is caused by apoC-I-induced inhibition of LPL

Studies in humans and mice have shown that increased expression of apolipoprotein C-I (apoC-I) results in combined hyperlipidemia with a more pronounced effect on triglycerides (TGs) compared with total cholesterol (TC). The aim of this study was to elucidate the main reason for this effect using human apoC-I-expressing (APOC1) mice. Moderate plasma human apoC-I levels (i.e., 4-fold higher than human levels) caused a 12-fold increase in TG, along with a 2-fold increase in TC, mainly confined to VLDL. Cross-breeding of APOC1 mice on an apoE-deficient background resulted in a marked 55-fold increase in TG, confirming that the apoC-I-induced hyperlipidemia cannot merely be attributed to blockade of apoE-recognizing hepatic lipoprotein receptors. The plasma half-life of [3H]TG-VLDL-mimicking particles was 2-fold increased in APOC1 mice, suggesting that apoC-I reduces the lipolytic conversion of VLDL. Although total postheparin plasma LPL activity was not lower in APOC1 mice compared with controls, apoC-I was able to dose-dependently inhibit the LPL-mediated lipolysis of [3H]TG-VLDL-mimicking particles in vitro with a 60% efficiency compared with the main endogenous LPL inhibitor apoC-III. Finally, purified apoC-I impaired the clearance of [3H]TG-VLDL-mimicking particles independent of apoE-mediated hepatic uptake in lactoferrin-treated mice. Therefore, we conclude that apoC-I is a potent inhibitor of LPL-mediated TG-lipolysis.